A great of deal of evidence now supports the concept that presenilins (PSs) are catalytic components of the proteolytic activity referred to as gamma-secretase. Using bioinformatics methods we have identified a family of proteins homologous to the polytopic membrane proteins, presenilin 1 and presenulin 2. This family, which we term Presenilin Homologues (PSHs), shows multiple structural similarities to the PSs. PSHs can often be aligned over their entire lengths with PSs, encompassing PS transmembrane (TM) domains 2-8. Additionally, PSHs and PSs contain identical aspartyl protease like active sites and many conserved residues that have been shown to be necessary for PS 1 dependent gamma-secretase cleavage of substrate. Based on these findings we hypothesize that PSHs and PSs comprise a novel family of polytopic membrane proteases. To explore this hypothesis we propose the following aims: (1) Develop tools to enable the study of PSH family members and initially characterize their mRNA expression pattern, protein products, and subcellular localization. (2) Determine jf the PSH proteins cleave any of the established gamma-secretase substrates. (3) Perform domain-swapping experiments between one or two PSH and PSi to determine if conserved PSH domains can be functionally substituted for PSi domains. (4) Determine if known gamma-secretase inhibitors that bind PS can bind PSH.